**6.2 Suitable pH**

8 Biogas

parameter. Although certain general properties are common from one environment to another, each environment may have its own unique population of bacteria, and associated microbial activities. Key operating factors which have a direct influence on the level and efficiency of biogas include volatile solids loading rate, digester temperature hydraulic

Marchaim (1992) noted that there is a close relationship between the biogas fermentation process and the temperature of the reactor. The higher the temperature, the more biogas is produced but when the temperature is too high, this can cause metabolic process to decline. Hobson et al., (1981) found biogas production to be greatest when the digester temperature was in the range of 32 to 400C. Hill (1982) also stated that digestion temperatures for optimum design all occur in the mesophilic range of 320C to 400 C. This work suggested that temperature beyond 400C has little effect on digester performance since the higher volumetric methane productivity is offset by the smaller digestion volume. As observed by the paper these lower temperatures also represent major savings in energy requirements when compared to thermophilic digestion (i.e. 600C). During the process of anaerobic biodigesiton in order to reach optimum operating temperatures (30–370C or 85–1000F), some measures must be taken to insulate the digester, especially in high altitudes or cold climates (VITA, 1980). Straw or shredded tree bark can be used around the outside of the digester to provide insulation. According to Carcelon and Clark (2002), anaerobic bacteria communities can endure temperatures ranging from below freezing to above 57.20C (1350 F), but they thrive best at temperatures of about 36.7 0C (980 F) (mesophilic) and 54.40C (1300 F) thermophilic. Bacteria activity, and thus biogas production falls off significantly between about 39.40C and 51.70C (103 0F and 125 0F) and gradually from 35 0C to 00 (95 0F to 32 0F). To optimize the digestion process, the digester must be kept at a consistent temperature as

The potential of thermophilic digester operating temperatures (> 550C) for anaerobic biogestion of livestock waste has been investigated by several researchers (Converse et. al., 1977; Hashimoto, et. al., 1979; Hashimoto, 1983; Hashimoto, 1984; Hill, 1985; Hill and Bolte, 1985; Hill et. al., 1986) with the technical feasibility being decided in favour of the process. Hill (1990) identified the advantages of thermophilic digestion over conventional mesophilic digestion as reduced hydraulic retention time (HTR), increased loading rate, and smaller physical reactors for identical waste amounts. The major disadvantage identified is the increased use of energy required to heat the feedstock and maintain digester operating temperature. Chen and Hashimoto (1981) however suggested that the development of heat

In cold climates, or during cold weather, optimal temperatures become very expensive to maintain, thus reducing the economic feasibility of the process of anaerobic biodigestion (Cullimore et al., 1985). In view of this, investigations have been conducted into the feasibility of anaerobic biodigesiton at lower temperatures. Stevens and Schulte (1979) thoroughly reviewed the literature regarding low–temperature digestion and found that methanogenesis occurs at temperatures as low as 40C, and that an increase in temperature from 40C to 250C dramatically increased the rate of methanogenesis. Cullimore (1982) reported results which indicated that as digester temperature was reduced from optimal

exchangers to recover energy in the effluent somewhat alleviated this advantage.

retention time, pH and carbon: nitrogen ratio (Vetter et al., 1990).

**6.1 Digester temperature** 

rapid changes will upset bacterial activity.

According to San Thy et al., (2003) biogas fermentation requires an environment with neutral pH and when the value is below 6 or above 8 the process will be inhibited or even cease to produce gas because of toxic effect on the methanogen population. The optimum for biogas production is when the pH value of the input in the digester is between 6 and 7. Increasing the amount of feedstock or a change in the fermentation material is likely to acidify the fermentation system because of the accumulation of volatile fatty acids (VFA). In this way pH can be used to indicate if the system is being overloaded. In the initial period of fermentation, as large amounts of organic acids are produced by the acid-forming bacteria, the pH in the digester may fall below 5 causing inhibition of the growth of the methanogenic bacteria and hence reduced gas generation (Da Silva, 1979). Acetate and fatty acids produced during digestion tend to lower the pH of the digester liquid (Marchaim, 1991). Hansen et al., (1998) stated that acetate-utilizing methanogens are responsible for 70% of the methane produced in biogas reactors.

Buren (1983) pointed out that the micro–organisms involved in anaerobic biodigestion require a neutral or mildly alkaline environment, as a too acidic or too alkaline environment will be detrimental. The work stated that a pH between 7 and 8.5 is best for biodigestion and normal gas production. The pH value for a digester depends on the ratio of acidity and alkalinity and the carbon dioxide content in the digester, the determining factor being the density of the acids. Buren (1983) noted further that for the normal process of digestion, the concentration of volatile acid measured by acetic acid should be below 2000 ppm, as too high a concentration will greatly inhibit the action of the methanogenic micro–organisms. Results of a study by Jantrania and White (1985) further confirm the foregoing. The study compared the performance of a number of digesters processing poultry wastes and found that the pH of the residue from digesters that failed were between 6.1 and 6.7, while the pH

Potentials of Selected Tropical Crops and Manure as Sources of Biofuels 11

The amount of gas produced depends on the slurry in the digester volume (Fulford, 1988). The digester volume is also related to the retention time measured in days and the loading rate, in terms of manure solids per unit liquid volume (San Thy et al., 2003). According to experiences in China, 97% of the total yield of gas from fermenting cattle manure will be produced in a period of 50 days at 350C. The hydraulic retention time (HTR) in anaerobic digesters is determined by calculating the number of days required for displacement of the fluid volume of the culture. At a given organic loading rate, the HTR is lower when using high water – content feeds than when using those containing less water (Fannin and Biljetina, 1987). The detention time is dependent on all the factors discussed above. Generally a retention time of between 30 and 45 days and in some cases 60 days is enough for substantial gas production (Clanton et al., 1985; Carcelon and Clark, 2002). A study by Hill (1982) found that detention times for digesters designed to produce maximum daily methane volume varied from 7.9 days for dairy waste to 14.8 days for poultry, and similar

None of the biological activities of anaerobic microorganisms, including their development, breeding and metabolism, requires oxygen. In fact, they are very sensitive to the presence of oxygen. The breakdown of organic materials in the presence of oxygen will produce carbon dioxide; in airless conditions, it produces methane (Buren, 1983; Voermans, 1985). Ferguson and Mah (1987) pointed out that methane–producing bacteria carry out the terminal step in the formation of biogas from the anaerobic decomposition of biomass. Methane is the final product of mineralizing the organic material in digesters and most anaerobic freshwater habitats. Most of the chemical energy in the starting materials (substrates) actually ends up in the methane released by these anaerobic bacteria. Ferguson and Mah (1987) noted further that in direct contrast, aerobic bacterial metabolism releases most of the chemical energy in the starting substrates by oxidizing them to carbon dioxide and water. Buren (1983) noted that if the digester is not sealed to ensure the absence of air. The action of the microorganisms and the production of biogas will be inhibited and some will escape. It is

There must be suitable moisture content of the feedstock as the microorganisms' excretive and other metabolic processes require water. The moisture content should normally be around 90% of the mass of the total contents (Buren 1983). Both too much and too little water are harmful, with too much water the rate of production per unit volume in the digester will fall, preventing optimum use of the digester. If the moisture content is too low, acetic acids will accumulate inhibiting the digestion process and hence production. Furthermore, a rather thick scum will form on the surface of the substrate. This scum may

The carbon:nitrogen (C/N) ratio expresses the relationship between the quantity of carbon and nitrogen present in organic materials. Materials with different C/N ratios differ widely

wide variations in loading rates existed between the two wastes.

therefore crucial that the biogas digester be airtight and watertight.

prevent effective mixing of the charge in the digester.

**6.5 Retention (or detention) time** 

**6.6 Air-tightness** 

**6.7 Moisture content** 

**6.8 Carbon: Nitrogen ratio** 

from the successful digester was 7.5. The digesters which stopped producing any appreciable amount of gas after 54 days had higher hydrogen sulfide content (over 200 ppm) than the successful digester.
